Generate random numbers with weighted probabilites

Question

I want to select a number randomly, but based on probability from a group of numbers; for example (2-6).

I'd like the following distribution:

6's probability should be 10%
5's probability should be 40%
4's probability should be 35%
3's probability should be 5%
2's probability should be 5%

Because you don't show what you've tried, what your research is and you question isn't clear at all. — Pieter, Jul 12 '13 at 12:15
it can be random for example when we throw a dice maximum times it comes 6 — user2340767, Jul 12 '13 at 12:15
I took a look at your previous Questions. Some hints: try to write with proper spelling (install a spell checker in your browser), use the [WYSIWYG tools](http://stackoverflow.com/editing-help) when writing, indent your code properly, write descriptive titles, read the guide [ask]. If your Q starts badly it's because you're failing in some (or all) of the previous hints. — brasofilo, Jul 12 '13 at 12:25

Silviu-Marian · Answer 1 · 2014-02-05T15:02:31.437

This is very easy to do. Watch for the comments in the code below.

$priorities = array(
    6=> 10,
    5=> 40,
    4=> 35,
    3=> 5,
    2=> 5
);

# you put each of the values N times, based on N being the probability
# each occurrence of the number in the array is a chance it will get picked up
# same is with lotteries
$numbers = array();
foreach($priorities as $k=>$v){
    for($i=0; $i<$v; $i++)  
        $numbers[] = $k;
}

# then you just pick a random value from the array
# the more occurrences, the more chances, and the occurrences are based on "priority"
$entry = $numbers[array_rand($numbers)];
echo "x: ".$entry;

score 3 · Answer 2 · answered Jul 12 '13 at 12:16

3

Create a number between 1 and 100.

If      it's <= 10       -> 6
Else if it's <= 10+40    -> 5
Else if it's <= 10+40+35 -> 4

And so on...

Note: your probabilities don't add up to 100%.

answered Jul 12 '13 at 12:16

Karoly Horvath

94,607
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score 2 · Accepted Answer · edited May 17 '23 at 02:13

The best you can do is generate a number between 0 and 100, and see in what range the number is:

$num = rand(0, 100);
if ($num < 10) {
    $result = 6;    
} elseif ($num < 50) { // 10 + 40
    $result = 5;
} elseif ($num < 85) { // 10 + 40 + 35
    $result = 4;
} elseif ($num < 90) { // 10 + 40 + 35 + 5
    $result = 3;
} else {
    $result = 2;
}

Be careful, if your total probability isn't equal to 1, then sometimes $result will be undefined.

Generate random numbers with weighted probabilites

3 Answers3